Industrial process controllers often use a variety of complex relay control systems to operate electrical equipment. Spring biased, self-sealing and magnetic latching relays, together with their associated control circuitry, are in widespread use throughout the industrial process control industry. Certain control operations, however, are so complex that presently-known relay-based topologies are impracticable or impossible. One example of such an operation is that control of a conveyor by three or more operators, each of whom needs to be able independently to start or stop the conveyor with the momentary closure of a foot switch. No practicable relay topology can implement such a system. In such situations, resort must be made to computer-based controllers.
Computer-based controllers are increasingly being utilized to implement otherwise impracticable industrial process control systems. Although effective, such controllers are expensive and require substantial time and expertise to program.
A generally applicable limitation on industrial relay control systems is that the equipment powered through the relay should come up unenergized when AC power is first restored after having been interrupted. There are two reasons for this limitation. One is that maintenance workers may be working on the equipment while the power is off. If power is inadvertently or unexpectedly reapplied, the workers can be seriously injured. The other reason is that the industrial equipment can be damaged by the voltage transients that generally accompany the reenergization of AC power distribution systems. Such transients can exceed the voltage ratings of the equipment, necessitating costly repairs. Although AC power outages can be minimized, they cannot be totally prevented. Accordingly, it is desirable in industrial applications to plan for the possibility of Ac power interruptions and to design control circuitry so that equipment comes up in its unenergized state when AC power is restored.
The nature of the control signals used also may impose limitations on the control system. For example, if momentary contact foot switches are used to control equipment, the control system must respond to an impulse, rather than to a continuous control signal. Latching relays are typically used in such applications, but, in case of a power interruption, such relays assume their last state when power is restored, with the attendant dangers mentioned above.
The nature of the industrial equipment being controlled can also dictate specialized limitations on the relay control system. The switching of AC power to certain machines in synchronization with the zero-crossings of the AC power signal is one such limitation. Since many industrial machines present a highly-inductive load to the AC power system, they present a very low impedance and draw a correspondingly large surge of current when fist energized. If such a machine is energized at the peak of the AC power signal, the current transient can exceed the current rating of the supply system, causing a power system circuit breaker to open. To assure an uninterrupted supply of power to the machinery, the switching of large industrial equipment should be synchronized with the zero-crossings of the AC power signal. Conventional relays are ill suited to switch such machinery since their contact closures are random with respect to the phase of the AC power signal.